Electrodes on an implantable sensing device are placed into the biological tissue of interest to monitor electrophysiological signals (current or voltage) or chemical/molecular signals (converted into current or voltage), as well as sometimes to deliver stimulating signals. Sensitivity of the electrode over time decreases due to cellular and acellular encapsulation as a result of the foreign body response around the device and electrode.
Recording and/or stimulating microelectrodes can be a critical enabling technology for both neuroscience and prosthetic treatment of spinal cord injury, amyotrophic lateral sclerosis, and limb amputation. Neural prostheses could greatly impact the treatment of these disorders by providing the means to interface the intention of a patient's mind and therefore restore some functional tasks [1-4]. However, the invasive nature of an intracortical neural probe requires high reliability and efficacy standards in order to justify the risk and cost of surgery. Failure due to tissue encapsulation is believed to be a major limitation to their widespread use and is also an issue for many other implantable biosensors [5-7].
Histological examination of intracortical devices has consistently shown that a glial scar forms around the probe tract [8-11]. Cellular components of the glial scar consist of activated microglia and hypertrophied astrocytes, and likely also include meningeal cells [12, 13], foreign body giants [8], and oligodendrocyte precursors [14]. These immunoreactive cells produce extracellular proteins that hinder local nerve regeneration [12, 15]. In addition, a neuronal “kill zone” has been reported around neural implants [11, 16].
After an injury, tissue encapsulation modifies the extracellular space as evidenced by mass transport [5, 17, 18] and impedance spectroscopy studies [19-21]. The injured tissue loses volume fraction and gains tortuosity [22]. Tissue encapsulation is also concomitant with a decrease in the signal quality of neural recordings in the brain and the periphery [19, 21, 23-27]. While electrode biofouling also contributes to a loss of perfonnance, tissue encapsulation has been shown to be a large factor [5].
The evidence has motivated the pursuit of several approaches to reduce tissue encapsulation around implantable devices, most notably surface modification [7, 28, 29] and local drug delivery [30-32]. However, a significant unmet need remains for microelectrodes that result in reduced tissue encapsulation and resulting loss of electrode performance.